There are several ways that semiconductor wafer containers are transported in a semiconductor fabrication facility (“fab”). A system for transporting a container is often referred to as an Automated Material Transport System (“AMHS”) or simply as a material transport system. A material transport system may refer to a part or all of the overall system. A fab may use only One type of AMHS throughout the fab, or there may be different types of AMHS in certain areas, or different types of AMHS for different transportation functions. Some of these AMHS types use vehicles to hold the container as it is being transported, such as a rail guided vehicle (RGV) or an automated guided vehicle (AGV). Material transport systems utilizing RGVs or AGVs require managing empty vehicles to arrange their arrival at sites where containers are to be picked up. Waiting for the arrival of such vehicles causes AMHS delays and the management of the vehicle movement increases the complexity of the AMHS. The same issues exist when moving containers with an Overhead Hoist Transport (OHT) system.
Conveyor systems are more efficient at moving containers within a fab without any, or a minimum number of, vehicle delays, and do not have to manage empty vehicles. Conveyors directly move the containers without any material or mechanical interface that comes between the conveyor surfaces and the container surfaces. Unless the conveyor is full, it is capable of immediately receiving a container for transport. For these, and other, reasons, conveyors may provide a very high throughput AMHS.
One example of a conveyor system is disclosed in U.S. Pat. No. 6,223,886, entitled “Integrated Roller Transport Pod and Asynchronous Conveyor,” which is owned by Asyst Technologies, Inc., and is incorporated in its entirety herein. The drive rail 12 includes a drive system, generally designated at 38 in FIG. 1A, for propelling a container 2 along the rails 12. The drive system 38 includes a plurality of separate drive assemblies 40. Each drive assembly 40 includes a plurality of drive wheels 42 which frictionally engage the underside of the container 2 to propel the container 2 along the drive rail 12 for a specific zone Z. As shown in FIG. 1A, the drive assemblies 40 are located along the rail such that the separation between the outermost drive wheels 42 of adjacent drive assemblies 40 is substantially equal to the spacing between the drive wheels 42 of the individual drive assembly 40. The drive wheels 42 project 15 upwardly from the drive rail housing such that it is the drive wheels 42 of the rail 12 which directly support the transport container 2. The wheels 42 are preferably mounted at approximately the same height to minimize tipping or rocking of the container 2 as it is moved along the rails 12. It is also known within the art to individually mount a passive wheel 43 between each drive wheel 42 (as shown in FIG. 1A).
It would be advantageous to provide a conveyor system that improves the performance of a conventional conveyor and reduces the costs of AMHS conveyor systems. The present invention provides such a conveyor.